Pressure sensors are used to detect variations in the pressure exerted on a surface or to measure the absolute value of the pressure exerted, and to convert the measured quantity into an electrical signal. A common use of such sensors is in the form of an acoustic transducer. Acoustic transducers detect changes in the pressure applied by sound waves and convert such changes into a varying electrical signal. Acoustic transducers may also be used in a reverse sense to convert electrical signals to pressure waves. When connected to an amplifier which drives a second transducer which reproduces the applied sound wave, an acoustic transducer can be made to function as a microphone.
One common type of acoustic transducer utilizes a diaphragm which moves in response to an applied sound wave. The diaphragm forms one plate of a two-plate capacitor. The movement of the diaphragm changes the separation of the capacitor plates, causing a variation in the capacitance of the capacitor. When used in conjunction with the appropriate circuitry, the change in capacitance produces an electrical signal which is proportional to the applied pressure. It is well known that the sensitivity of such a pressure sensor increases with a decrease in the thickness of the diaphragm. This is because a thinner diaphragm has less inertia and can respond more rapidly to small pressure variations. In addition, because the size of the transducer scales with the thickness of the diaphragm, reducing the thickness of the diaphragm leads to both a more sensitive and a smaller device.
FIG. 1 is a side view of a prior art micro-machined pressure sensor or acoustic transducer 100 formed by processing techniques used in the semiconductor industry. As shown in the figure, transducer 100 is formed from a silicon substrate 102 into which is etched an aperture 104 for entrance of a pressure wave. The pressure wave impacts diaphragm 106, causing the diaphragm to move in response to variations in the pressure applied by the wave. Diaphragm 106 forms one plate of a two-plate capacitor. The second plate is formed by a perforated electrode (not shown) located above diaphragm 106. Movement of diaphragm 106 causes the capacitance of the two-plate capacitor to vary, producing a changing electrical signal.
There are two primary methods currently used in the semiconductor industry to fabricate diaphragm 106 for use in an acoustic transducer. The first method is based on the diffusion mechanisms which occur in boron gas phase doping processes. This method uses boron tri-chloride (BCl.sub.3) as the source gas to produce a highly doped p+ layer which serves as an etch stop. While this method is capable of producing diaphragm films of thickness greater than one micron (1 .mu.m), it has not proved useful in producing thinner films. This is because it has not been possible to reproducibly grow highly doped films of thickness less than one micron in this manner.
The second method used for producing diaphragms of the type shown in FIG. 1 is to implant Boron ions into a thin film. The use of such a method in fabricating a differential pressure sensor is discussed in U.S. Pat. No. 5,332,469, entitled "Capacitive Surface Micromachined Differential Pressure Sensor", issued Jul. 26, 1994. However, such a method of forming the diaphragm is not useful for fabricating very thin diaphragms because implantation of the Boron ions produces stress in the film which causes buckling and cracking of the diaphragm.
Thus, both of the currently used methods for making diaphragms for pressure sensors and acoustic transducers are incapable of reliably producing diaphragms having a thickness less than one micron. As a result, the methods cannot be used to produce pressure sensors or acoustic transducers which have increased sensitivity and reduced size compared to currently available devices. Another disadvantage of the two currently used methods for forming the sensor diaphragm is that they result in the formation of a parasitic reverse biased p+ n diode (element 108 of FIG. 1) which acts to electrically isolate diaphragm 106 from substrate 102. This increases the power required to operate the device and reduces the responsivity of the two-plate capacitor, making the sensor less sensitive.
What is desired is a method for producing pressure sensors and acoustic transducers having a high sensitivity and reduced size which overcomes the disadvantages of currently used techniques.